The present invention relates in general to implantable cardiac stimulation devices, including bradycardia and anti-tachycardia stimulation devices, defibrillators, cardioverters and combinations thereof that are capable of measuring physiological data and parametric data pertaining to implantable medical devices. More particularly, this invention relates to a system and method for automating detection of atrial capture and determination of an atrial pacing threshold in an implantable cardiac stimulation device.
Implantable cardiac stimulation devices (such as pacemakers, defibrillators, and cardioverters) are designed to monitor and stimulate the heart of a patient that suffers from a cardiac arrhythmia. Using leads connected to a patient""s heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to detected cardiac events which are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart""s regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, the shape, the location, and the frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program (which is responsible for the operation of the implantable device) can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Modern implantable devices have a great number of adjustable parameters that must be tailored to a particular patient""s therapeutic needs. One adjustable parameter of particular importance in stimulation devices is the stimulus energy (i.e., the pulse amplitude and pulse width) which can be programmed to new values in response to changes in capture threshold. xe2x80x9cCapturexe2x80x9d is defined as a cardiac depolarization and contraction of the heart in response to a stimulation pulse. When a stimulation pulse stimulates either a patient""s atrium or ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a xe2x80x9ccapture thresholdxe2x80x9d which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should be achieved at the lowest possible energy setting yet provide enough of a safety margin so that should a patient""s threshold increase, the output of an implantable stimulation device (i.e. the pacing stimulus energy) will still be sufficient to maintain capture. Dual chamber stimulation devices may have different atrial and ventricular pacing stimulus energies that correspond to different atrial and ventricular capture thresholds, respectively.
The earliest stimulation devices had a predetermined and unchangeable pacing stimulus energy, which proved to be problematic because the capture threshold is not a static value. The capture threshold also may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around stimulation device lead tips within several months after implantation may cause an increase in the capture threshold. To avoid loss of capture, the earliest stimulation devices were preset to deliver pacing pulses at the maximum energy available. As a result some patients experienced discomfort because of the high level of stimulation. Furthermore, such stimulation pulses consumed extra battery resources, thus shortening the useful life of a stimulation device.
When programmable stimulation devices were developed, the pacing stimulus energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted stimulation device via telemetry or via a magnet applied to a patient""s chest. The particular setting for the stimulation device""s pacing threshold was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient""s capture threshold, from the patient""s medical history, and from a listing of the patient""s medications. This improvement in adjustable pacing stimulus energy permitted programming to lower values that tended to conserve battery energy and extend the useful service life of the stimulation device.
Also, patients who experienced discomfort due to excessively high stimulus energy pulses could have the stimulus energy safely decreased thus, lessening the incidence of surgical revision of the pacing system. While the adjustable pacing stimulus energy feature proved to be superior to the previously known static stimulus energy, some significant problems remained unsolved. In particular, when a patient""s capture threshold changed, the patient was forced to visit the medical practitioner to adjust the pacing stimulus energy accordingly.
To address this pressing problem, manufacturers have developed advanced stimulation devices that are capable of determining a patient""s capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This approach, referred to herein as xe2x80x9cautocapturexe2x80x9d, improves the patient""s comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the stimulation device""s battery life by conserving the energy used for stimulation pulses.
A common technique used to determine whether capture has been effectuated is to monitor the patient""s cardiac activity and to search for presence of an xe2x80x9cevoked responsexe2x80x9d following a stimulation pulse. The evoked response is an electrical event that is the response of the heart to the application of a stimulation pulse thereto. The patient""s heart activity is typically monitored by the stimulation device by keeping track of the stimulation pulses delivered to the heart and by examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue(myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by the generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by the generation of an R-wave (sometimes referred to as the xe2x80x9cQRSxe2x80x9d complex when viewed on an ECG strip).
When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the atrium (hereinafter referred to as an xe2x80x9cA-pulsexe2x80x9d), any response sensed by atrial sensing circuits of the stimulation device immediately following application of the A-pulse is presumed to be an evoked response that evidences capture of the atria.
However, it is for several reasons very difficult to detect a true atrial evoked response. First, a high energy A-pulse may obscure the evoked response signal, making it difficult to detect and identify. Second, the signal sensed by the atrial sensing circuitry immediately following the application of an A-pulse may be not an evoked response, but noisexe2x80x94either electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point where an electrode of the lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as the A-pulse, across the interface. Unfortunately, because the atrial evoked response is sensed through the same lead electrode through which the A-pulse is delivered, the resulting polarization signal formed at the electrode can corrupt the evoked response sensed by the atrial sensing circuits. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and other variables, many of which are continually changing over time.
In each case, the result may be a false positive detection of an atrial evoked response. Such an error leads to a false atrial capture indication, which in turn leads to missed heartbeats-a highly undesirable and potentially a life-threatening situation. Another problem results from a failure by the stimulation device to detect an atrial evoked response that has actually occurred. In this case, a loss of atrial capture is indicated when atrial capture is in fact present-also an undesirable situation that will cause the stimulation device to unnecessarily invoke the atrial pacing threshold determination function and result in higher than necessary stimulus energy values.
Because of the problems previously stated regarding the test for atrial capture verification and automatic threshold tests, currently available stimulation devices do not have this capability. As a result, many medical practitioners manually conduct atrial capture verification tests during periodic follow up examinations. These periodic follow-up examinations are performed by the medical practitioner after initial implantation and configuration of the stimulation device to determine whether the therapy delivered by the device is having the desired effect and to verify the proper operation. Capture verification and pacing threshold assessment is typically performed by the medical practitioner using an external programmer for controlling the stimulation device functions in conjunction with a surface electrocardiogram (ECG) device.
However, this common capture verification and pacing threshold assessment procedure is a time consuming and complex task requiring significant attention and effort on the part of the medical practitioner. The medical practitioner must spend a significant amount of time placing and subsequent removal of ECG electrodes, and configuring the ECG system for the patient""s individual characteristics. The practitioner must also manually examine the ECG readout and analyze the cardiac waveform to determine whether capture is present both during initial capture verification and during the pacing threshold determination tests.
It would thus be desirable to provide a system and method for enabling the stimulation device to automatically perform atrial capture verification and atrial pacing threshold determination without a medical practitioner""s involvement. It would also be desirable to enable the stimulation device to perform the atrial capture verification and atrial pacing threshold determination without requiring dedicated circuitry and/or special sensors. It would further be desirable to maintain a record of atrial pacing threshold determination in the stimulation device so that a medical practitioner can verify the proper operation of the stimulation device by examining the record.
The disadvantages and limitations discussed above are overcome by the present invention. In accordance with the invention, a system and method are provided for automating (1) verification of proper atrial capture affected by atrial pacing pulses generated by a patient""s implantable cardiac stimulation device, and (2) dynamic adjustment of the device""s atrial pacing stimulus energy if and as necessary. The system and method of the present invention do not require use of special dedicated circuitry or special sensors to implement the automated procedures. All of the aforesaid advantages and features are achieved without incurring any substantial relative disadvantage.
The present invention is directed towards the pacing pulse generating portion of an implantable cardiac stimulation device (i.e., a bradycardia pacemaker or the pacing portion of a combination ICD/pacemaker device).
A preferred embodiment of the stimulation device includes a control system for controlling the operation thereof, a set of leads for receiving atrial and ventricular signals and for delivering atrial and ventricular stimulation pulses, a set of sense amplifiers for sensing and amplifying the atrial and ventricular signals, and pulse generators for generating the atrial and ventricular stimulation pulses. In addition, the stimulation device includes memory for storing operational parameters for the control system, and for storing data acquired by the control system for later retrieval by the medical practitioner using an external programmer. The stimulation device also includes a telemetry circuit for communicating with an external programmer.
Preferably, the stimulation device of the present invention is a dual chamber rate-responsive device with atrial tracking modes (such as, DDD and DDD(R)) capable of switching modes to at least a non-tracking mode (such as, DDI and DDI(R)). Accordingly, an activity sensor is also included for sensing when the patient is at, or near, rest.
In a preferred embodiment, the control system periodically performs an atrial capture verification test and an atrial pacing capture threshold assessment test. The frequency with which these tests are to be performed is preferably a programmable parameter set by the medical practitioner using an external programmer when the patient is examined during an office visit or remotely via a telecommunication link. The appropriate testing frequency parameter will vary from patient to patient and depend on a number of physiologic and other factors. For example, if a patient is on a cardiac medication regimen, the patient""s atrial capture threshold may fluctuate thus requiring relatively frequent testing and adjustment of the atrial pacing threshold.
In order for the capture verification and threshold assessment tests to work properly, the patient preferably should be at, or near, rest such that a stable atrial rhythm can be monitored by the stimulation device. Thus, prior to initiating atrial capture verification, the control system detects whether the patient is at, or near, rest using the patient activity sensor. If the patient is not at or near rest, the control system waits for a predetermined period of time before attempting to initiate the test again.
When the control system finally determines that the patient is at or near rest, the atrial capture verification test is initiated by first assessing the intrinsic atrial rate or P-P interval. The intrinsic atrial rate must be greater than the base rate such that the intrinsic, or native, P-waves are detectable. When the stimulation device is pacing, the Base Rate must be temporarily programmed to a lower value to allow the intrinsic atrial rate to emerge from the pacing rate. The reprogramming of the Base Rate may be performed in decrements of 5 to 10 ppm until a minimum lower rate, not less than 30 ppm is obtained. The temporary lower rate can be limited by the medical practitioner through the use of the programmer. If the rate of 30 ppm (or the minimum prescribed lower Base Rate of the stimulation device) is reached without the emergence of an intrinsic rhythm, the capture assessment test is automatically terminated.
With the emergence of an intrinsic atrial rate, greater than the Base Rate, the mode of operation is changed from the atrial tracking modes (such as, DDD and DDD(R)) to a non-tracking mode (such as, DDI and DDI(R)). This temporary mode change is necessary to avoid occurrence of a Pacemaker Mediated Tachycardia (PMT) during the testing process. A PMT is a type of arrhythmia that sometimes occurs in VDD or DDD type stimulation devices, in which sensing of retrograde P-waves occurs in the atrium and triggers the ventricle. Retrograde conduction occurs in response to ventricular pacing, causing atrial contraction (i.e. a P-wave). Sensing of this P-wave causes the ventricle to again be stimulated, completing an xe2x80x9cendlessxe2x80x9d loop and thus subjecting the patient to PMT. Switching of the stimulation device into DDI mode eliminates the triggered response in the ventricle, thus preventing the occurrence of PMT.
After the mode switch, the control system monitors and measures the patient""s average P-wave interval over a short period of time, and then defines an expected P-wave xe2x80x9cwindowxe2x80x9d of predetermined duration in which P-waves are expected to occur. The control system next generates an A-pulse at a predetermined prematurity time interval prior to the next expected P-wave window and thereafter monitors the expected P-wave window to determine whether a P-wave occurs within the window. The lack of a P-wave within that window indicates that an evoked P-wave occurred as a response to the A-pulse immediately following the A-pulse (i.e., outside the expected intrinsic P-wave window). Thus, if a P-wave is not detected during the window, atrial capture is present. If atrial capture is thus verified, the control system switches the stimulation device back to original atrial tracking mode (i.e., DDD or DDD(R)) and ends the atrial capture verification test.
The presence of a P-wave within the window, on the other hand, indicates that there was no P-wave immediately following the A-pulse and thus no atrial capture. In this case, the control system needs to perform the atrial pacing threshold assessment test to set a new atrial pacing threshold to re-establish atrial capture.
The control system sets atrial stimulation (i.e. the A-pulse) level below the previous atrial pacing level (or at a level that is expected to be below the patient""s capture threshold), generates the A-pulse and monitors the window for a P-wave. If a P-wave is again detected within the window, then the control system increments the A-pulse level and then generates the A-pulse at the higher level while monitoring the window. This process continues until a P-wave is no longer present during the window interval.
The control system continues to monitor the window for a predetermined number of pacing cycles to ensure that no P-waves occur within the window, and then records the atrial pacing stimulus energy at the current A-pulse output level as the threshold value and, optionally, adds an additional safety margin to the A-pulse threshold value. The control system records the atrial pacing threshold, the atrial stimulation levels, and other test-related data in the memory, and then switches the stimulation device back to original atrial tracking mode before ending the test.
The incremental atrial pacing threshold test of the present invention significantly differs from previously known approaches because atrial stimulus output is initially set lower than the current threshold and progressively increased until capture occurs, while previously known approaches set initial atrial output at a high level and then decrement until capture is lost. The progressive output increase approach is advantageous over prior approaches because less electrical energy is consumed during the testing process and, moreover, because the window observed by the control system is not xe2x80x9cswampedxe2x80x9d by high output level pulses.
In an alternate embodiment, the method of incrementally increasing the A-pulse level can also be used in an atrial capture system that employs an xe2x80x9cevoked responsexe2x80x9d detection window following a stimulus, wherein only a paced, or evoked, P-wave in the detection window indicates capture, as is well known in the art.
Optionally, if the patient suffers from sinus bradycardia that is accompanied by retrograde conduction, the expected P-wave window is set to at least a predetermined portion of the cardiac cycle, and the control system then searches for retrograde P-waves within the window. Similarly, presence of retrograde P-waves within the window indicates loss of capture, while lack of retrograde P-waves confirms capture. If necessary, the atrial pacing threshold is assessed and set in the same manner as previously described.
The system and method of the present invention thus automatically verify atrial capture and, when necessary, automatically determine a proper atrial pacing threshold of the patient, without requiring dedicated or special circuitry and/or sensors.